The present invention is directed to a DC-DC converter and, more particularly, a DC-DC converter having over-voltage protection to protect a load of the DC-DC converter.
DC-DC converters are extensively employed in many electronic devices such as desktop computers, laptops, and servers, as well as more sophisticated equipment, such as telephone central office switching offices, cellular telephone base stations, data storage systems, etc. In particular, it is common practice to employ a DC-DC converter to supply a highly-regulated voltage to integrated circuits, such as a microprocessor, in such computing equipment.
A type of DC-DC converter which is extensively employed employs two switches connected in bucking relationship. One is connected to a supply voltage line and the other to ground or an equivalent return path. A common node connects the two switches, and the voltage of the common node is filtered and supplied to the load. The two switches are alternatively turned on and off based on a pulse width modulated signal. The voltage of the output node is varied and controlled by the width of the pulses.
FIG. 1 is an illustration of a prior art DC-DC converter which employs metal oxide field effect transistors (MOSFETS). A control circuit 21 provides a pulse width modulated signal to MOSFET driver 22 for the MOSFETS 13 and 14. MOSFET 13 is connected to a substantially constant voltage, for example, to voltage supply node 10. MOSFET 13 is also connected to the common node 15 (denoted in FIG. 1 as a “phase node”). MOSFET 14 connects the common node 15 to ground. The MOSFET driver 22 provides shoot-through protection so that MOSFET 14 never conducts at the same time as MOSFET 13, because that would short the supply voltage on node 10 to ground. The voltage supplied to common node 15 is filtered by inductor 16 and capacitor 17 to stabilize the output voltage on OUTPUT node 18. The output voltage supplied to node 18 is for supplying power to load 19. Feedback line 60 feeds the output voltage back to the control circuit 21, which responds to the output voltage by adjusting the width of the pulses and thus controls the output voltage. Capacitor 12 minimizes the voltage ripple on node 10 caused by the switching current through the MOSFET 13. A sensed current signal is connected to the control circuit 21 through resistor 20.
In many cases, the load energized by such a converter is quite expensive, and such a DC-DC converter has an unfortunate failure mode in which the switch connecting supply voltage to the common node fails closed. In that event, the supply voltage is connected directly to the load, thus damaging or destroying the load
FIG. 2 is a schematic based on oscilloscope traces of voltages during failure of switch 13, in which it fails in the conducting mode. Curve 55 represents the voltage of the common (or “phase”) node 15. The left side of the figure illustrates the normal situation in which common node 15 is alternatively connected to node 10 and to ground through switches 13 and 14, respectively. The right side of the figure illustrates the failure mode, in which it is only connected to the voltage supply node 10. Curve 58 illustrates the voltage of the output node 18. As can be seen from the figure, the voltage of the output node 18 rises continuously toward the voltage of the voltage supply node 10.
To prevent the type of failure mode illustrated in FIG. 2, various circuit arrangement for clamping or otherwise preventing the voltage on output node 18 from exceeding a preset limit have been proposed.
U.S. Pat. No. 6,873,191 provides a circuit for clamping the voltage of the common node and hence the output voltage. To accomplish this, the common node is connected through a resistor to the gate of the MOSFET that connects the common node to ground. When the voltage of the common node increases, the resistive connection to the gate of the MOSFET increases the voltage of the gate and causes the MOSFET to conduct. This defeats the shoot through protection, and shorts the supply voltage to ground. Thus, the load is protected from excessive voltage.
U.S. Pat. No. 6,731,486 has a voltage protection circuit connected to the gate of the transistor which connects the common node to ground. In the event of an over-voltage, the voltage protection circuit causes the transistor to conduct. Like the preceding patent, this defeats the shoot through protection, and the supply voltage is shorted to ground.
U.S. Pat. No. 5,751,531 employs a clipping device such as a zener diode to limit the voltage applied to an output. As with the two preceding patents, this is done by providing a low resistance path to ground.
U.S. Pat. No. 5,335,132 has a zener diode which breaks down in the event of an over-voltage condition, whereby the operating point of the circuit is shifted.
U.S. Pat. No. 5,091,818 has an over-voltage protection circuit which has a voltage clamping transistor to clamp the power supply voltage to a voltage of less than the rated voltage.
U.S. Pat. No. 4,727,308 has an over-voltage protection circuit in which excess voltage is bypassed to ground through a flywheel field effect transistor. This results in a surge in input current, which opens a fuse in the input to the converter.
U.S. Pat. No. 3,737,725 is a circuit over-voltage protector. The circuit has a zener diode which breaks down in the event of an over-voltage. Heavy current through the zener diode causes a circuit breaker or fuse to open.
While the preceding patents have interesting features, it is believed that an improved over-voltage protection principle is needed for a DC-DC converter.